In biological purification of water or waste water the water is passed through some type of reactor wherein micro-organisms are utilized to convert impurities existing in the water to harmless end products such as carbon dioxide and water. The purification i.a. can be performed under supply of air (aerobically) or without supply of air (anaerobically). In order to increase the efficiency of the purification a high content of active micro-organisms in the process commonly is aimed at so that said organisms are not allowed to escape with the purified water, either by the micro-organisms being allowed to grow in suspension in the reactor and being separated from the water in a separation step after the reactor and being returned to the reactor (e.g. the activated sludge process), or by some type of carrier material being introduced into the process, on the surfaces of which the micro-organisms can grow as a biofilm and thus can be maintained in the process (the biofilm process).
The biofilm process has a number of advantages as compared with the activated sludge process and other processes with suspended biomass. Among other things higher loads can be used and the processes are substantially less sensitive to variations and disturbances. Most conventional biofilm processes are based on the purification reactor being packed with carrier material in the shape of filler bodies or blocks which are fixed to be stationary in the process. These process embodiments have the drawback that there is a risk of the carrier material being stopped by biomass or other particulate material and that dead zones may be established in the process where the contact between the water and the active micro-organisms is very bad.
In another type of biofilm process a carrier material is utilized which is held in suspension and movement in the process. The carrier material with micro-organisms growing thereon is maintained in the process by outgoing water being allowed to pass through a strainer or grid having an opening diameter or slot width which is so small that the carrier material cannot pass therethrough. The advantage of this type of process is i.a. that the risk for stopping of the carrier material and the establishment of dead zones is substantially reduced. Different types of carrier material are available for this type of process: foamed rubber pieces (EP-A-0 142 123), rings of non-woven plastic material (Haikangijutu Vol.29(4): pp 64-69, 1987), and pieces of extruded plastic hoses the surface of which is enlarged by mechanical processing (Haikangijutu vol. 29(4): pp 64-69, 1987), intermixing of material which provides a rough surface (Water Environment Research vol. 64(7): pp 884-889), or introduction of inside partitions and outside fins (WO-A-91/11396). These materials have in common that they have a density which is close to the density of water so as to be easily held in suspension. Although processes with carrier elements of these types often can be operated at high loads all prior art embodiments of carrier elements have, however, drawbacks which substantially limit the capacity of the process, particularly in aerobic processes wherein the supply of oxygen to the working micro-organisms is of decisive importance for the purification efficiency. In foamed rubber pieces and rings of non-woven material the pores often are blocked by growing biomass so that the active surface in contact with waste water and air will be reduced. The carrier elements which have been produced by extruding a plastic hose are small, &lt;1 cm, in order that the surface per volume will be large. Also in these elements the passages often are blocked, and also in case this does not happen it has been found that the supply of oxygen to the biofilm which in most cases is located on the inside surfaces of the carriers is heavily limiting for the process. Also, it has been found that the small size of these carrier elements causes difficult practical problems due to the fact that they will be caught by foam which is produced in the processes, and thus can be carried away from the process by the foam or the wind. Furthermore, there will often arise problems in treating waste water containing suspended particles, e.g. fibers, by the small openings in the strainers or grids holding back the carrier elements being blocked. For e.g. waste water carrying fibers in the forest industry these small carrier elements have been found to be directly unsuitable for this reason.
So far it has not been possible to overcome these problems by making larger carrier elements because it has not been possible as far as prior art embodiments are concerned to provide a large carrier with a large surface for the biofilm, protected against wear, without the efficiency of the process being heavily reduced by a still greater oxygen limitation of the biofilm than in available small carriers.